Hi Uwu,
My only suggestion in how to tell him is to be honest and open about what you feel, when you feel it and why. I suspect that the longer you betray yourself by trying to force yourself to do something you don't want to do, the worse it will be in the long run.
I too was sexually assaulted when I was young, and then in my late teens I too had a boyfriend that i loved very much, but could not, for the life of me, relax when we were mucking around in the bedroom. I would freeze completely and be unresponsive. And to be honest, it was him that pointed it out ..... but not in a nice way at all. I had not told him at that stage that I had been abused, so he had no idea at all that I was struggling, and couldn't understand why I was being the way I was. He ended up cheating on me and we had a big fight. It was only after we cleared the air and he told me why he wasn't satisfied with me, that I then told HIM what had happened to me, and how it had left me feeling. He was more understanding after that ..... but we broke up anyway later on. Mainly because I just couldn't trust him any more. But also because I got drink one night and tried to 'pay him back' by cheating on him. The whole thing was messy and looking back, probably quite unavoidable if I had just been honest from the start.
Anyway, the truth, I have found, is always best. Even if it does sting him a little at first. At least you will have been honest with him, and true to yourself. Sex is a delicate, but important issue, that needs to be dealt with in any intimate relationship.
I wish you all the best in dealing with this issue. Take care.
